1. Field of the Invention
The present invention relates to an exposure apparatus which projects the pattern of an original onto a substrate via a projection optical system to expose the substrate to light, and a device manufacturing method.
2. Description of the Related Art
A full-plate exposure apparatus such as a stepper or a scanning exposure apparatus such as a scanner can be employed to manufacture a device such as a semiconductor device.
FIG. 9 is a view showing the schematic arrangement of an exposure apparatus. A light source LS can be, for example, a superhigh pressure mercury lamp (g-line (wavelength: about 436 nm) or i-line (wavelength: about 365 nm)), a KrF excimer laser (wavelength: about 248 nm), or an ArF excimer laser (wavelength: about 193 nm). A light beam emitted by the light source LS is supplied to an illumination optical system IL. The illumination optical system IL illuminates a set region on an original (also called a reticle or mask) under set illumination conditions. A fine circuit pattern to be transferred onto a photosensitive agent applied on a substrate WF is formed on the original RT. The pattern formed on the original RT is projected onto the substrate WF via a projection optical system PO. Along with the recent trend toward the micropatterning of integrated circuits, a very high demand has arisen for increasing the alignment accuracy between the original RT and the substrate WF in substrate exposure. To ensure high productivity, it is also necessary to complete alignment measurement in a short period of time.
An example of an alignment measurement system is the TTL (Through The Lens) measurement system as shown in FIG. 9. The TTL measurement system images a projected image of a slit pattern, which is formed on an original reference plate RFP that is flush with the original RT held by an original stage RS, in the vicinity of a reference slit on a substrate reference plate WFP arranged on a substrate stage WS. Light transmitted through the reference slit is detected by a sensor IS. The original slit pattern may be formed not on the original reference plate RFP but on the original RT. For the sake of simplicity, the following description assumes that the original slit is formed on the original reference plate RFP.
More specifically, as shown in FIG. 2A, a plurality of marks are formed on the original reference plate RFP to allow measurement at a plurality of image heights in the exposure region. Each mark includes a plurality of slits. Light having passed through the slit of the mark on the original reference plate RFP is transmitted through the reference slit on the substrate reference plate as shown in FIG. 2B, and detected by the sensor IS. Measurement is performed by monitoring a detection signal from the sensor IS while changing the relative positional relationship between the mark on the original reference plate RFP and the mark on the substrate reference plate WFP. At this time, focus measurement is performed by monitoring a detection signal from the sensor IS while moving the mark on the original reference plate RFP and/or the mark on the substrate reference plate WFP along the optical axis of the projection optical system PO. The relative position between the original stage RS and the substrate stage WS in the horizontal direction is measured by monitoring a detection signal from the sensor IS while moving the mark on the original reference plate RFP and/or the mark on the substrate reference plate WFP along a plane perpendicular to the optical axis of the projection optical system PO.
FIG. 3 illustrates an example of the detection signal from the sensor IS in the TTL measurement system, which assumes a point at which the amount of light is maximum as a best point (BP). The waveform of the detection signal is influenced by the illumination conditions (e.g., the effective light source distribution and amount of light) of the illumination optical system IL shown in FIG. 9.
Along with the recent micropatterning, various types of illumination conditions such as illumination which uses a small annular zone ratio, very-low-σ illumination, and dipole illumination have come to be set for the illumination optical system IL. The above-described TTL measurement system is required to be always capable of alignment measurement even under these various types of illumination conditions.
Unfortunately, a detection signal exhibiting a small overall light amount as illustrated in FIGS. 4A and 4B leads to deterioration in the accuracy of determining a point at which the amount of light is a maximum. A detection signal having a plurality of peaks as illustrated in FIG. 4C results from erroneous detection at a very high probability. High-accuracy alignment is also difficult when using detection signals obtained under an illumination condition under which illuminance nonuniformity occurs or that under which the pole balance is bad, because the maximum amount of light does not match the best point.